1. Field of the Invention
The present invention relates to an objective lens actuator for an optical recording/reproducing apparatus such as an optical storage drive for optically recording and reading information by a laser beam focused on an optical recording medium such as an optical disk.
2. Description of the Prior Art
In recent years, intense interest has been shown towards optical recording/reproducing apparatus as a unit of equipment which is capable of recording, reproducing and erasing information on the recording medium.
In such optical recording/reproducing apparatus, a laser beam emitted from a semiconductor laser is focused on a rotating optical disk and the reflected light or transmitted light is detected to regenerate signals representing pieces of information or data recorded on the optical disk. To this end, the apparatus needs a tracking control for projecting a laser beam onto a desired position on the optical disk, and a focus control for accurately focusing the laser beam on the optical disk. In general, these controls are accomplished by displacing an objective lens by means of an objective lens actuator.
One such known objective lens actuator is disclosed in Japanese Patent Laid-open Publication No. 63-50926. The disclosed objective lens actuator includes, as reillustrated here in FIGS. 7 through 9, a wire member 1 fixed at its central portion to an objective lens holder 2, a pair of parallel spaced plate springs 4 supported at one end by a stationary support member 3 and joined at the opposite end with the opposite ends of the wire member 1 for supporting the objective lens holder 2, and an objective lens 5 fixedly mounted on the objective lens holder 2 with its optical axis spaced from, and substantially parallel with, an axis of the wire member 1. The objective lens holder 2 has a substantially cylindrical shape and holds a longitudinal axis substantially in common with the wire member 1. A focusing coil 6 is wound around the periphery of the cylindrical objective lens holder 2. Four substantially rectangular tracking coils 7 are disposed on an outer surface of the focusing coil 6 and equidistant from one another in the circumferential direction of the objective lens holder 2.
A magnetic circuit base 8 includes a pair of arcuate, diametrically opposed first arcuate magnetic yokes 8a, 8a integral with the magnetic circuit base 8, and a pair of arcuate, diametrically opposed second magnetic yokes 8b, 8b disposed radially outwardly of the first magnetic yokes 8a, 8a, respectively, and mounted on the magnetic circuit base 8 with a pair of arcuate, diametrically opposed permanent magnets 9, 9 disposed between the corresponding second magnetic yokes 8b, 8b and the magnetic circuit base 8. The first magnetic yokes 8a are received in a pair of axial holes 10 formed in the objective lens holder 2. The stationary support member 3 is secured to the magnetic circuit base 8 in such a manner that the axis of the wire member 1 is substantially aligned with a common axis of the arcuate first and second yokes 8a, 8b and arcuate magnets 9.
With this construction, when a tracking control current flows through the tracking coils 7 of the objective lens actuator, the objective lens holder 2 is subjected to a rotational force which in turn causes the wire member 1 to twist, thereby turning the objective lens holder 2 about the axis of the wire member 1. The tracking control of the objective lens 5 is thus performed. When a focus control current flows through the focus control coil 6, the objective lens holder 2 is subjected to a driving force acting in a direction parallel to the axis of the wire member 1. The driving force causes the parallel spaced plate springs 4 to flex, displacing the objective lens holder 2 in a direction parallel to the axis of the wire member 1. The focus control of the objective lens 5 is thus performed.
The wire member 1 of the conventional objective lens actuator has a relatively large spring constant in a direction circumferential about its own axis (this direction being referred to as "tracking direction") and a relatively small spring constant in a direction perpendicular to its own axis (this direction being referred to as "focusing direction"). Yet, a free selection of the ratio between these two spring constants is substantially impossible. Since the displacement of the objective lens 5 in the tracking direction relies on the twist of the wire member 1, the wire member 1 having a relatively large spring constant in the same direction has a high primary resonance frequency. In this case, resonance vibration takes place at a very high frequency level which is no longer possible to lower or dampen by means of a viscoelastic member. An acceleration during a high speed access or an external vibration or shock acts in a direction perpendicular to the axis of the wire member 1, the wire member 1 which is supported on the free ends of the cantilevered parallel plate springs 4 is freely bendable depending on a force exerted on the objective lens holder 2. Due to this bending of the wire member 1, the central axis of a laser beam which is incident upon the objective lens 5 is misaligned with, or tilts with respect to, the optical axis of the objective lens 5.
The foregoing extremely high resonance frequency level and the misalignment between the laser beam and the optical axis of the objective lens 5 exert negative influence on the servo characteristics of the objective lens actuator. Thus, an accurate recording/reproducing of signals is difficult to perform.